1. Field of the Invention
The present invention relates to an optical switching element capable of polarizing incident light into two directions, and an optical switching device and an image display apparatus each using the optical switching element.
2. Description of the Related Art
In recent years, importance of a display as a display device of image information is increasing. As an element for the display and, further, as elements for optical communication, an optical memory device, an optical printer, and the like, development of an optical switching element operating at high speed is in demand. Conventionally, elements of this kind include an element using a liquid crystal, an element using a micromirror, an element using a diffraction grating, and the like. FIG. 1 shows an example of the element using a liquid crystal. FIGS. 2A and 2B to FIG. 5 show examples of the elements using micromirrors. FIGS. 6A and 6B show an example of the element using a diffraction grating.
An optical switching element using a liquid crystal (FIG. 1) includes two polarizing plates 101a and 101b, two glass substrates 102a and 102b, transparent electrodes 103a, 103b, 103c, and 103d, and a liquid crystal 104 sealed between the glass substrates 102a and 102b. The optical switching element performs switching operation by applying a voltage across the transparent electrodes 103a, 103b, 103c, and 103d to control the directions of liquid crystal molecules, thereby rotating a plane of polarization.
The liquid crystal has, however, response of only about a few milliseconds at the highest and therefore has a problem such that the liquid crystal has a characteristic of low response. Consequently, it is very difficult to apply the liquid crystal to optical communication, optical calculation, optical memory device such as a hologram memory, optical printer, and the like requiring fast response. Since the optical switching element using the liquid crystal needs two polarizing plates, there is also a problem that efficiency of light utilization decreases. Further, since the liquid crystal is not resistive to strong light, light having high energy density such as a strong laser beam cannot be switched. Particularly, in the case of using the optical switching element for a display, higher image quality as compared with that of optical switching elements of recent years is requested. The optical switching element using the liquid crystal of current conditions starts to be insufficient with respect to accuracy in gradation display.
As for the optical switching element using a micromirror, there are already a number of examples typified by the DMD (Digital Micromirror Device) of Texas Instruments Incorporated (U.S.). The examples of the DMD can be broadly divided into two kinds with respect to the structure; a structure in which a micromirror is supported by one side (FIGS. 2A and 2B and FIG. 3) and a structure in which a micromirror is supported by both sides (FIGS. 4A and 4B and FIG. 5). Micromirror driving methods include a method using electrostatic attraction, a method using a piezoelectric device, and a method using a thermal actuator. In spite of difference in structure, driving method, and the like, the function of switching incident light by controlling the angle of a micromirror is a very simple one.
A micromirror of a type using electrostatic attraction will be described here as an example. The driving principle of the micromirror is as follows. In the case where a micromirror 105 is supported by one side (FIGS. 2A and 2B and FIG. 3), by giving a potential difference between the micromirror 105 and a drive electrode 106, the electrostatic attraction is generated to make the micromirror 105 tilt. When the given potential difference is eliminated, the micromirror 105 returns to its original state by spring force of a hinge 105a supporting the micromirror 105.
In the case where the micromirror is supported by both sides (FIGS. 4A and 4B and FIG. 5), the same potential difference is created between a micromirror 108 and two electrodes 107a and 107b facing the micromirror 108. From such a state, for example, by decreasing a voltage applied to one, 107a, of the electrodes and increasing a voltage applied to the other electrode 107b, unbalance is caused between the electrostatic attraction generated between the electrode 107a and the micromirror 108 and the electrostatic attraction generated between the electrode 107b and the micromirror 108, thereby making the micromirror 108 tilt.
Light is switched as follows. In the case of the micromirror supported by one side (FIGS. 2A and 2B and 3), in a state where the micromirror 105 is not tilted with respect to incident light P100, reflection light travels in a direction P101. In a state where the micromirror 105 tilts with respect to the incident light P100, reflection light travels in a direction P102. In the case of the micromirror supported by both sides (FIGS. 4A and 4B and FIG. 5), similarly, in a state where the micromirror 108 tilts in one direction with respect to incident light P100, reflection light travels in a direction P103. In a state where the micromirror 108 tilts in another direction with respect to the incident light P100, reflection light travels in a direction P104.
The response is, however, about a few microseconds in many cases. It cannot be said the speed is high enough. In order to perform gradation display by a digital control using time division, one micromirror is necessary per pixel in an image, that is, a two-dimensional micromirror array is necessary. It is considered that the demand on the higher image quality is increasing more and more. In this case, manufacturing of a necessary two-dimensional micromirror array will become very difficult. In an optical switching element using the micromirrors, a light polarizable angle (angle difference between two reflection light) is about twice as large as a mechanical mirror runout angle. However, in the case of using the optical switching element for a display, to make the contrast high, the angle difference between the two reflection light P103 and P104 has to be set large. It causes a problem that the response deteriorates more.
In the optical switching device using a diffraction grating (FIGS. 6A and 6B), as disclosed in Translated National Publication of Patent Application Ser. No. Hei10-510374, a ribbon-shaped movable mirror 109a is moved by a quarter of the wavelength of the incident light P100 by electrostatic attraction generated by making a potential difference between the movable mirror 109a and a lower electrode 110a, so that an optical path difference corresponding to the half of the wavelength is caused between a ribbon-shaped stationary mirror 109b and the movable mirror 109a, thereby generating diffraction light. The reflection light is switched between the direction of zero-order diffraction light P105 and the direction of primary diffraction light P106. In this case, by controlling the optical path difference within the range to the half wave, the intensity of the primary diffraction light P106 can be controlled. In an optical switching device using a diffraction grating, only by moving a very light ribbon-shaped mirror by a short distance, light can be switched. Consequently, the optical switching device has fast response (about tens nanoseconds) and is suitable for high-speed switching.
The primary diffraction light is, however, generated with certain angles in two directions symmetrical to the optical axis of the zero-order diffraction light. Consequently, in order to use the primary diffraction light, a complicated optical system for collecting light traveling in the two directions to a single light is necessary. To make the light diffract, at least two ribbon-shaped mirrors are necessary per pixel. In order to increase the efficiency of light utilization, four or more, actually, six ribbon-shaped mirrors are necessary. In a light valve (spatial light modulator) in which a necessary number of pixels of the ribbon-shaped mirrors six per pixel are formed in an array, it is desired that a reflection face of the stationary mirror 109b and a reflection face of the movable mirror 109a are flush with each other to avoid generation of the primary diffraction light without applying voltage to the electrodes. In practice, however, the reflection faces are not easily adjusted to be flush with each other. Fine adjustment is therefore necessary to make all of the reflection faces flush with each other by applying a low voltage to each of lower electrodes 110a and 110b. 